The ability of noble-metal nanowires to guide propagating surface plasmon polariton (SPP) modes with wavelengths that are considerably shorter than the corresponding free-space wavelength (e.g., see Sanders, A. W. et al., “Observation of Plasmon Propagation, Redirection, and Fan-Out in Silver Nanowires,” Nano. Lett., 6:1822-1826 (2006); Nelayah, J. et al., “Direct Imaging of Surface Plasmon Resonances on Single Triangular Silver Nanoprisms at Optical Wavelength using Low-Loss EFTEM Imaging,” Opt. Lett., 34:1003-1005 (2009); Ditlbacher, H. et al., “Silver Nanowires as Surface Plasmon Resonators,” Phys. Rev. Lett., 95:257403 (2005); Dickson, R. M. & Lyon, L. A., “Unidirectional Plasmon Propagation in Metallic Nanowires,” J. Phys. Chem. B, 104:6095-6098 (2000); Takahara, J. et al., “Guiding of a One-Dimensional Optical Beam with Nanometer Diameter,” Opt. Lett., 22:475-477 (1997)) has driven considerable interest in SPP-based applications in areas such as light harvesting, nanoscale imaging, and spectroscopy (see Brongersma, M. L. & Kik, P. G., “Surface Plasmon Nanophotonics,” Springer (2007); Shalaev, V. M. & Kawata, S., “Nanophotonics with Surface Plasmons,” Elsevier (2007); Maier, S. A., “Plasmonics: Fundamentals and Applications,” Springer (2007); Engheta, N., “Circuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials,” Science, 317:1698-1702 (2007); Fang, Y. et al., “Remote-Excitation Surface-Enhanced Raman Scattering Using Propagating Ag Nanowire Plasmons,” Nano. Lett., 9:2049-2053 (2009); Hutchison, J. A. et al., “Subdiffraction Limited, Remote Excitation of Surface Enhanced Raman Scattering,” Nano. Lett., 9:995-1001 (2009). However, most of these applications require the ability to combine noble-metal nanowires with other nanostructures.
On-demand fabrication of nanowire-based devices that incorporate plasmonic elements and other nanomaterials, such as quantum dots or dielectric nanoparticles, remains a challenging problem. Strategies for creating such devices have generally relied on random assembly (Knight, M. W. et al., “Nanoparticle-Mediated Coupling of Light into a Nanowire,” Nano. Lett., 7:2346-2350 (2007); Akimov, A. V. et al., “Generation of Single Optical Plasmons in Metallic Nanowires Coupled to Quantum Dots,” Nature, 450:402-406 (2007)) or self-assembly (Curto, A. G. et al., “Unidirectional Emission of a Quantum Dot Coupled to a Nanoantenna,” Science, 329:930-933 (2010)), followed by searching for individual structures with the desired components and configuration. Thus, the ability to more easily assemble preselected nanostructures at predetermined locations on a metal nanowire would be beneficial for the creation of a broad range of devices.
Another issue that arises in nanophotonic devices that involve noble-metal nanowires is the inherent difficulty of coupling light into SPP modes from the far field when using conventional techniques. Momentum-matching constraints only allow efficient far-field coupling for light that is nearly parallel to the nanowire axis, a condition that is often difficult to achieve using conventional techniques, particularly over a broad range of wavelengths. This difficulty has led to the development of approaches for near-field coupling into nanowires (e.g., see Yan, R. et al., “Direct photonic-plasmonic coupling and routing in single nanowires,” Proc. Nat. Acad. Sci. USA, 106:21045-21050 (2009), but such approaches generally rely on random assembly.